1. Field of the Invention
This invention relates to protective coating systems for components exposed to high temperatures, such as the hostile thermal environment of a gas turbine engine. More particularly, this invention is directed to a beta-phase nickel aluminide overlay coating whose grain structure is modified to improve oxidation resistance.
2. Description of the Related Art
Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of engine components must correspondingly increase. Significant advances in high temperature capabilities have been achieved through the formulation of nickel and cobalt-base superalloys. Nonetheless, when used to form components of the turbine, combustor and augmentor sections of a gas turbine engine, superalloys can be susceptible to damage by oxidation and hot corrosion attack and may not retain adequate mechanical properties. For this reason, turbine, combustor and augmentor components are often protected by an environmental and/or thermal-insulating coating, the latter of which is termed a thermal barrier coating (TBC) system.
Environmental coatings that have been widely employed to protect gas turbine engine components include overlay coatings such as MCrAlX (where M is iron, cobalt and/or nickel, and X is yttrium or another rare earth element), and diffusion aluminide coatings, particularly those containing platinum aluminide (Ni(Pt)Al) intermetallic. The aluminum content of these materials provides for the slow growth of a strong adherent and continuous aluminum oxide layer (alumina scale) at elevated temperatures, which protects the coating and its underlying substrate from oxidation and hot corrosion. As apparent from their names, overlay and diffusion coatings are distinguishable in terms of the processes by which they are formed and the thickness of the zone of chemical interaction that occurs within the substrate surface beneath the coating. This zone, referred to as a diffusion zone (DZ), results from the interdiffusion between the coating and substrate. The diffusion zone beneath an overlay coating is typically much thinner than the diffusion zone created within a diffusion bond coat. Diffusion aluminide coatings are also distinguished from overlay coatings, in that the former consists of intermetallic compounds that form as a result of interdiffusion, while the latter can be multi-phase, containing phases such as gamma (γ) and beta (β) nickel aluminide structures if the substrate is a nickel-base superalloy.
Ceramic materials such as zirconia (ZrO2) partially or fully stabilized by yttria (Y2O3), magnesia (MgO) or other oxides, are widely used as thermal barrier coatings (TBC's), or topcoats, on gas turbine engine components. To be effective, TBC's must strongly adhere to the component surface and remain adherent throughout many heating and cooling cycles. The latter requirement is particularly demanding due to the different coefficients of thermal expansion between TBC materials and the superalloys typically used to form turbine engine components. TBC systems capable of satisfying the above requirements have generally required a bond coat, typically formed of one or both of the above-noted diffusion aluminide and MCrAlX coatings. In addition to protecting the bond coat and underlying substrate from oxidation and hot corrosion, the alumina scale that grows on diffusion aluminide and MCrAlX coatings serves to chemically bond a ceramic layer to the bond coat. A thermal expansion mismatch exists between metallic bond coat materials, the alumina scale and ceramic layer, which results in stresses at their interfaces. Over time, microcracking and damage increase, eventually leading to spallation of the TBC.
In view of the above, it can be appreciated that bond coats have a considerable effect on the spallation resistance of the TBC, and therefore TBC system life. Consequently, improvements in TBC life have been sought through modifications to the chemistries of existing bond coat materials. Other types of bond coat materials have also been proposed, such as beta-phase nickel aluminide (NiAl) overlay coatings that have also found use as environmental coatings. The NiAl beta phase is an intermetallic compound that exists for nickel-aluminum compositions containing about 30 to about 60 atomic percent aluminum. Notable examples of NiAl coating materials are disclosed in commonly-assigned U.S. Pat. No. 5,975,852 to Nagaraj et al., U.S. Pat. No. 6,291,084 to Darolia et al., U.S. Pat. No. 6,153,313 to Rigney et al, and U.S. Pat. No. 6,255,001 to Darolia. These NiAl alloys, which preferably contain a reactive element (such as zirconium) and/or other alloying constituents (such as chromium), have been shown to improve the adhesion of a ceramic TBC layer, thereby increasing the service life of the TBC system.
In addition to modifications to their chemistry, the effect of the surface finish of diffusion aluminide and MCrAlY bond coats on TBC spallation resistance has also been investigated, as evidenced by U.S. Pat. No. 4,414,249 to Ulion et al. with respect to MCrAlY overlay coatings, and commonly-assigned U.S. Pat. No. 6,340,500 to Spitsberg and co-pending U.S. patent application Ser. No. 09/524,227 to Spitsberg with respect to diffusion aluminide coatings. Ulion et al. disclose that TBC service life can be improved by polishing the surface of a peened and heat-treated MCrAlY overlay bond coat. The benefit of peening is said to be increased density of the bond coat. The Spitsberg patent and patent application teach that the benefit of improving the surface finish of a diffusion aluminide bond coat is that the resulting modified surface morphology of the bond coat eliminates or at least reduces oxidation and oxidation-induced convolutions at the alumina-bond coat interface. The Spitsberg patent further teaches that peening and then heat treating a diffusion aluminide bond coat can significantly improve TBC service life, particularly if the bond coat does not undergo recrystallization during heat treatment. In contrast, the pending Spitsberg application teaches that TBC service life is improved by recrystallizing a diffusion aluminide bond coat to eliminate the original grain boundaries, which is believed to have the effect of creating more stable grains and reducing the quantity of refractory phases at the grain boundaries.
The mechanism by which TBC spallation initiates can depend on the type of bond coat used. Spallation of TBC deposited on one of the aforementioned beta-phase NiAl overlay bond coats has been observed to occur by delamination of the alumina scale from the bond coat or TBC delamination from the alumina scale. However, the mechanism by which spallation initiates from an NiAl overlay bond coat differs from MCrAlX and diffusion aluminide bond coats as a result of differences in chemistry, microstructure and mechanical properties. For example, NiAl overlay bond coats are believed to exhibit a different spallation mechanism than diffusion aluminide bond coats as a result of having higher creep resistance and flow or yield strengths at elevated temperatures.
Though having the above-noted advantages, TBC service life on NiAl overlay bond coats containing zirconium and/or chromium has been found to be sensitive to Zr and Cr content. Therefore, improvements in TBC service life deposited on NiAl overlay bond coats would be desirable. However, possible modifications in chemistry, microstructure and mechanical properties that might achieve an improvement must take into account the unique characteristics of NiAl overlay coatings, including the mechanism by which TBC spallation is initiated on an NiAl overlay bond coat.